Heart rate variability (HRV) has been used as a noninvasive index of parasympathetic tone. Based on our previous data on parasympathetic studies, we proposed the hypothesis that HRV response to both the sympathetic and parasympathetic stim-ulation/blockade display a variance which is dependent upon the initial level of autonomic tone and the intensity of stimulus. This study will test that hypothesis. Measurements of HRV during stepwise changes in above conditions will be plotted to test the hypothesis.